There are a wide variety of data communication standards suitable for carrying data information among network devices. Certain protocols support providing power through network cables to remote devices such as VOIP (Voice Over Internet Protocol) telephones. Thus, in addition to supporting data communications, the network cables also support providing power to remote network devices. Providing power to remote devices through a network cable alleviates the user from having to additionally tether the remote device to a separate power source such as a 115 volt wall outlet.
Protocols developed for the purpose of supplying power to remote devices (for VOIP and other applications) through network cables typically include an initial discovery phase. During the discovery phase, a switch determines whether or not to provide power through the network cable to the remote device. For example, discovery typically includes receiving some form of feedback from the remote device indicating that it is safe to send power over the network cable to power the remote device. After it is deemed appropriate, the switch then powers the remote device through the network cable.
In addition to providing power to remote devices over a network cable, protocols have been developed to terminate power supplied through the network cable when the remote device is unplugged or turned off. In one application, a switch providing power to the remote device monitors a data stream from the remote device at a link layer to identify when a corresponding link with the remote device has been terminated as a result of a being unplugged from the network cable. According to a conventional protocol, even though a remote device has been unplugged from a network cable supplying power, the switch continues to provide power (during a blind window) through the cable for a significant amount of time which in some cases is up to 1.4 seconds. This blind window of supplying power through the network cable power occurs largely because a link down-state machine monitoring the link at a link layer has built-in hystreresis. For example, the link-down state machine does not flag that a link is technically ‘down’ until a considerable amount of time has passed after a cable has been unplugged from a remote device. Historically, this feature of not immediately depowering a remote device through a network cable has been used to eliminate false linkdowns.